1. Field of the Invention
The present invention relates to ground fault equipment protection, and, more particularly, to ground fault circuit interrupters.
2. Description of the Related Art
Ground fault circuit interrupters (GFCI, also known as a ground fault interrupter or GFI) are designed to protect from electrical shock by interrupting a circuit when there is a difference in the currents in the hot and neutral wires. Such a difference indicates that an faulty diversion of current from the wires is occurring, for example, a leakage current to ground from the hot wire. Such a current might be flowing in the ground wire, such as a leakage current from a motor or from capacitors. More importantly, the current diversion may occur because a person has come into contact with the hot wire and is being shocked. When a circuit is functioning normally, all the return current from an electrical load flows through the neutral wire, so the presence of a difference between hot and neutral currents represents a malfunction which in some circumstances can produce a damaging, dangerous or even lethal shock hazard.
Ground fault circuit interrupters are generally required by the electrical code for receptacles in bathrooms, some kitchen receptacles, some outside receptacles, and receptacles near swimming pools. Scenarios which led to these code requirements include dropping a hair dryer or a portable radio into a bathtub with a person, while the appliance is still connected to a source of electrical power, causing electrocution of the person. A typical circuit breaker interrupts the circuit at 20 amperes (A), but it takes only about 100 milliamperes (mA) or less to electrocute a person in such a scenario. A household ground fault circuit interrupter is designed to detect currents of a few milliamperes and trip a circuit breaker at the receptacle or at the circuit breaker panel to remove the shock hazard. Industrial applications can include greater voltages and/or currents which can be an even greater shock or fire hazard.
A ground fault circuit interrupter can have a “Test” button which causes a small difference between hot and neutral currents to test the ground fault circuit interrupter. For example, the test button can apply the 120 volt AC (alternating current) supply across a 14.75 kilo-ohm (kΩ) resistor, producing a current of 8 mA where, for example, a requirement for a ground fault circuit interrupter can be that it trip when there is 6 mA of leakage current, and more specifically, the ground fault circuit interrupter de-energizes a circuit within an established period of time (25 milliseconds) when a current to ground (ground-fault leakage) exceeds some predetermined value (6 mA, for Class A GFCI) which is far less than that required to operate the overcurrent (overload) protection device (breaker or fuse) of the supply circuit. The ground fault circuit interrupter can also include a reset button to use after it has been tripped.
An example of a ground fault circuit interrupter circuit construction is hot and neutral wires are passed through a sensing coil so that the currents in the two wires at any instant are traveling in opposite directions, giving a net zero current in the coil if the two currents are equal. Since a current carrying wire produces an AC magnetic field external to the wire, a non-zero current induces a voltage in the sensing coil. The sensing coil can be wound longitudinally to capture more of the wire's magnetic field and therefore can be a more sensitive net current detector. The current difference that is detected is relatively small. The ground fault circuit interrupter circuit can be designed to trip when there is more than 6 mA of leakage current out of the normal pathway. For example, the neutral return has 6 mA less current than the hot wire, out of up to a 20 A load. The voltage in the sensor coil is rectified and applied to the input of a sensitive comparator. The comparator activates a trip coil which opens both the hot and neutral wires thereby interrupting the circuit. If all circuits were wired with correct polarity, opening the hot wire is sufficient, but since it is not uncommon for the circuits to be accidentally wired backward, it is prudent to interrupt both hot and neutral wires.
Because of the sensitivity of the detection of current differences, GFCIs are prone to nuisance tripping in certain applications. Further, a single GFCI can be used to protect several circuits. Unfortunately, GFCIs can be disconnected to eliminate the nuisance tripping, and thereby eliminate ground fault protection for one or more circuits.
Additionally, a ground fault circuit interrupter can fail in a way that permits the device to be reset and pass current without providing ground fault protection. Many people do not understand how a GFCI works and hence may believe that protection is being provided when, in fact, the device has failed. An article in IAEI News (International Association of Electrical Inspectors) has reported a high failure rate of GFCIs. The protective circuitry in a GFCI is vulnerable to voltage spikes such as those caused by lightning and high-voltage switching. It is possible for the GFCI to eventually fail to provide ground fault protection while still providing power for equipment, tools and appliances. A proper test sequence can help assure that the GFCI is still providing ground fault protection; however, such a test sequence is operator dependent.
In general, GFCI protection equipment is subject to undetected failures that inhibit their function. Random component failures caused by lightning and other electrical surges, abuse, along with normal wear and tear, all can cause component failures that can render a GFCI inoperable and thus incapable of performing its safety function. This is particularly true in freeze protection application where equipment may be unused other than during the winter months. For example, in pipe heater trace freeze protection and similar applications, the pipe heater trace equipment is typically unused in the warm months. During these warm months, maintenance of piping and accessories such as valve seals can damage the heating cable. Additionally, a lightning strike can damage a GFCI for the heater trace equipment. When the heater trace equipment is operated for the first time in the cold months, due to the inadvertent damage to the equipment caused by maintenance and/or the lack of ground fault protection due to the lightning strike, a shock and/or fire hazard can exist.
What is needed in the art is an apparatus and method for automatically monitoring ground fault apparatus.